1. Field of the Invention
The present invention relates to an apparatus for measuring the number or sizes of micro impurities such as fine dust or bacteria floating in ultrapure water.
2. Description of the Prior Art
Ultrapure water is water which has a specific resistivity of 16 M.OMEGA..multidot.cm (25.degree. C.) and contains impurities such as fine particles, organic materials, inorganic materials or bacteria only in units of ppb (1/1,000 of the ppm). Ultrapure water is indispensable to the manufacture of IC semiconductors. In particular, since the pattern size of current LSIs is as small as 1 .mu.m, ultrapure water must not contain micro impurities having a size of 0.1 .mu.m or more.
Methods of measuring micro impurities (to be referred to as impurities for brevity hereinafter) floating in ultrapure water currently include the light-blocking method, the light-scattering method, the laser-scattering method, the filter method, and the electrical pulse method.
Of these methods, the light-blocking method, the light-scattering method, and the laser-scattering method which utilize light provide only a low precision and cannot measure or detect impurities having a size of 0.5 .mu.m or less or transparent impurities such as dead bacteria. Although the filter method which measures the impurities trapped by a microscope allows measurement of impurities having a size of 0.1 .mu.m or less, sampling and measurement procedures require much labor and time.
In the electrical pulse method, a change in resistance which is obtained when an impurity passes through a small hole in a detector is obtained as a voltage pulse. The electrical pulse method theoretically allows measurement of impurities having a size of 0.1 .mu.m or less and allows easy sampling and measurement. However, it sometimes happens that the electrolyte or ultrapure water causes electrolysis between the electrodes, bubbles become attached to the surfaces of the electrodes, and the detection sensitivity is signifcantly degraded. When a high current is flowed to improve detection sensitivity, electrolysis becomes more vigorous to present danger. Furthermore, since the interior of a detector is kept at a reduced pressure to draw by suction ultrapure water through the small hole in the detector, dissolved gases in the electrolyte in the detector or a manometer appear in the form of bubbles and adversely affect the operation of the manometer.